An isoprene which is a main material of a synthetic rubber etc. is usually obtained by extraction distillation of isoprene contained in a C5 fraction which is extracted by an ethylene cracker of an ethylene center.
In the process of extraction distillation of the isoprene contained in a C5 fraction, by removing the cyclopentadiene from the C5 fraction by dimerization (forming dicyclopentadiene), then each removing light fractions such as pentanes and pentenes and heavy fractions such as pentadienes (including dicyclopentadiene and 1,3-pentadiene) and acetylenes by two distillation towers, and further removing diolefins (including 1,3-pentadiene) and remaining acetylenes by a next extraction distillation tower, then distilling the residue, it is possible to efficiently obtain isoprene from the tower bottom.
At this time, C5 raffinate which is the extracted residual oil is obtained, but it is possible to return the C5 raffinate to the ethylene center and mainly utilize it as a gasoline base material or raw material of the ethylene cracker. Further, the removed dicyclopentadiene, 1,3-pentadiene, etc. may be utilized as a raw material of a resin etc.
In this regard, each a concentration of the isoprene, dicyclopentadiene, 1,3-pentadiene, etc. in the C5 fraction is generally constant, so demand for products using each as a raw material does not necessarily match each a ratio of concentration. Accordingly, sometimes the remainder is returned to the C5 raffinate. Therefore, the concentration of diolefins in the C5 raffinate which is returned to the ethylene center sometimes fluctuates at the level of several tens of percent.
Further, the C5 fraction contains sulfur-containing ingredients in a concentration of several weight ppm to several hundred weight ppm. Therefore, the C5 raffinate which is the extracted residual oil also contains sulfur-containing ingredients in several weight ppm to several hundred weight ppm.
Therefore, when using a C5 raffinate as the raw material of an ethylene cracker, there are the problems that if the C5 raffinate contains large amounts of sulfur-containing ingredients and diolefins as explained above, the catalyst in the diene removing tower which is installed at the refining section of the ethylene plant at the ethylene center remarkably deteriorates, the amount of consumption of hydrogen at the refining section greatly increases, and the profitability of the ethylene plant ends up deteriorating. Further, most of diolefins are high in polymerizability and their polymers easily form initiating substances causing fouling in the cooling pipes, so if diolefins are included in large amounts, there is also the problem that an increase in the frequency of cleaning of the cooling pipes is invited. Further, the catalyst in the diene removing tower end up being poisoned by the sulfur-containing ingredients, so if the sulfur-containing ingredients are included in large amounts, there is also the problem that a need arises for regenerating or replacing the poisoned catalyst and the running costs of the catalyst deteriorates.
For this reason, a C5 raffinate which contains diolefins and sulfur-containing ingredients in large amounts has problems in terms of both quality and cost, so at the present time cannot be utilized as a hydrocarbon material, in particular a hydrocarbon material of an ethylene cracker, and is being burned as fuel.
On the other hand, due to the recent rising interest in environmental issues, the increase in carbon dioxide has become a concern. The need for effective utilization of crude oil has been rising. Therefore, it is desirable to utilize the C5 raffinate as a hydrocarbon material rather than burning it.
For this reason, to use a C5 raffinate which contains diolefins at a concentration of several tens of percent and contains sulfur-containing ingredients in an amount of several weight ppm to several hundred weight ppm as a hydrocarbon material for, in particular, an ethylene cracker, it is necessary to remove the sulfur-containing ingredients as much as possible and hydrogenate double bonds of the diolefins and olefins etc. as much as possible.
As opposed to this, for example, Patent Document 1 and Patent Document 2 disclose a method of selective hydrogenation of thermally cracked gasoline which uses a reaction tube packed with solid metal catalysts for selective hydrogenation by a first stage catalyst comprised of a palladium-based catalyst and a second stage catalyst comprised of a cobalt-molybdenum-based catalyst.
However, with the method described in Patent Document 1 and Patent Document 2, there are the problems that the hydrogenation is performed under a high pressure and the productivity is inferior. Further, the Patent Document 1 and Patent Document 2 do not describe anything about the effect of desulfurization. Further, they do not describe anything regarding the lifetimes of the catalysts which are used at the time of selective hydrogenation.